A cardinal premise of leftist thought is that cultural traits run deep. They reach down, past behavior, to unconscious values and concepts, shaping how we think. I went to graduate school in the 1980s, when critical race theorists and postcolonialists talked about “Western ways of knowing.” Western thought, they said, treats the world as a set of inert objects to be analyzed and consumed, while non-Westerners see the world as fluid and animated. A difference as fundamental as that wasn’t to be erased, multiculturalists insisted. As the West draws the non-Western and developing worlds further into its orbit, they cautioned, it must avoid the crime of cultural imperialism.

This is why we must take the outrage over Amy Wax’s remarks at the National Conservatism Conference in Washington earlier this month with a grain of salt. Ms. Wax, a law professor at the University of Pennsylvania, argued that the U.S. should reduce immigration from non-Western countries because those migrants aren’t likely to assimilate as smoothly into American society as Western immigrants do. There is too much “cultural incompatibility” between the U.S. and developing world. And because those countries are mostly nonwhite, Ms. Wax continued, basing immigration policy on cultural matching “means in effect”—and only in effect—that “our country will be better off with more whites and fewer non-whites.”

That line has formed the core of the case against Ms. Wax. A Vox.com writer called it “an outright argument for white supremacy.” Minority student groups at Penn demanded she be relieved of all course assignments. Penn Law Dean Ted Ruger labeled her statement “repugnant.”

What her detractors haven’t addressed, however, is Ms. Wax’s assertion of the deep acculturation that makes people who they are. This must be respected. The great divide Ms. Wax identifies is between peoples that have passed through the Enlightenment and peoples that haven’t. Immigrants from countries that don’t have a tradition of individual rights, free markets and fair elections must undergo a firm and steady induction if those mores are to sink into their souls. Social conservatives and identity-politics leftists agree on this: People can’t easily drop their heritage and adopt another one.

It is liberals and libertarians who think that migration is a smooth process. They imagine a world of free and flexible people who pick and choose the elements that will form their characters. Neither conservatives nor progressives trust this cosmopolitan faith. They know that culture molds character.

Ms. Wax’s great sin in the eyes of the left wasn’t her recognition of cultural differences and incompatibilities. It was, instead, her frank declaration of the West’s cultural superiority. She mentioned “the homegrown conditions and failures that hold countries back” and the “persistently chaotic conditions in the Third World.” These are things you don’t say in polite society. Mr. Ruger turned Ms. Wax’s factual statements about conditions in developing countries into “a bigoted theory of white cultural and ethnic supremacy.” A petition on Change.org started by a Penn grad bears the heading “Fire Racist Penn Law Professor, Amy Wax” and has more than 30,000 signatories.

This outspoken praise for the West is anathema to the left, but not because the left hates the idea of cultural superiority. Far from it. The left most definitely believes in cultural superiority—but the kind that runs the other way. To them, the West isn’t a story of the advancement of rights and scientific knowledge, as Ms. Wax believes. It is a record of exploitation, enslavement, colonialism, environmental devastation and imperialism against suffering and benign non-Western peoples. When we speak of the West, the U.S. and whites, we must confess guilt.

This is a dogma in academia, advocacy groups and the Democratic Party. No, it’s a taboo. It has extraordinary force, too, having intimidated Republicans for decades. Amy Wax violated it. She’s not afraid. The left knows it, and if she isn’t punished, she may inspire others.